This invention relates to the separation of gaseous mixtures. It is particularly concerned with the separation of gaseous mixtures comprising carbon dioxide and methane to form a product gas having a reduced concentration of carbon dioxide.
It is well known to purify gas mixtures comprising at least one hydrocarbon and carbon dioxide by absorbing the carbon dioxide in a suitable liquid. The capital and running costs of such purification plant tend to increase rapidly with increasing concentration of carbon dioxide. Moreover, liquids employed as absorbents tend to be corrosive, and thus relatively high maintenance costs are typically associated with such plants. Accordingly, other methods of separating hydrocarbons from carbon dioxide have been proposed. Among such other methods are those involving liquefaction of the gas mixture and fractional distillation of the liquid to produce a product vapour fraction relatively lean in carbon dioxide.
Where a mixture of methane and carbon dioxide contains relatively small proportions of carbon dioxide, say, less than 1% by volume the carbon dioxide is typically removed by adsorption, for example using a molecular sieve. It has been alternatively proposed to subject the mixture to distillation at a pressure of about 650 psia after preliminary treatment to remove any such impurities as hydrogen sulphide and mercaptans that are present in the incoming gas mixture and to reduce the temperature of the mixture to a point at which it is fully saturated or at least partially liquefied. By performing such a distillation the bulk of the gas may be removed from the top of the distillation column free from carbon dioxide while a liquid fraction including 12% by volume of carbon dioxide is removed from the bottom of the column. The phase diagram of the carbon dioxide-methane system at 650 psia is such that at carbon dioxide concentrations above 12% by volume, solid carbon dioxide will be deposited. Thus, no more than 12% by volume of carbon dioxide is included in the liquid fraction, and hence if the process is used to treat a mixture containing more than about 1% by volume of carbon dioxide there will be a substantial loss of product gas which will increase with increasing carbon dioxide concentration in the incoming gas mixture. The process, therefore, is not suitable for treatment of carbon dioxide-methane mixtures containing more than about 1% by volume of carbon dioxide.
Fractional distillation has also been employed to separate a carbon dioxide product from natural gas relatively rich in carbon dioxide (e.g. containing more than 20% by volume of carbon dioxide). In this instance, in order to avoid deposition of solid carbon dioxide, the distillation is performed at a higher pressure, typically 750 psia. However, at such pressures, owing to the nature of the phase relationships within the methane-carbon dioxide system, it is impossible to produce pure methane, and typically the liquid fraction contains substantially pure carbon dioxide and the vapour fraction about 12% carbon dioxide. This process is thus unsuitable if it is desired to obtain a methane product relatively free of carbon dioxide.
It has been proposed to provide greater operational flexibility in such fractional distillation processes by adding to the gaseous mixture of carbon dioxide and methane a substance that inhibits the deposition of solid carbon dioxide. (See, for example, U.S. Pat. specification No. 4,318,723). By this means it is possible to take a feedstock that contains a relatively high proportion of carbon dioxide and obtain a product methane fraction containing less than 12% by volume of carbon dioxide. However, such measures are not entirely satisfactory as they involve substantial capital and operating cost to separate and recycle the inhibitor. There is also added expense in providing and storing the inhibitor.
An improved process for removing high concentrations of carbon dioxide from a gas mixture comprising carbon dioxide and methane was proposed by G. C. Schianni at the Natural Gas Processing and Utilisation Conference of 1976. In this process a raw gas containing 75% mole per cent of carbon dioxide is cooled and is then passed to a high pressure column, operating at about 52 bar (i.e. above the critical pressure of methane). The bottom product consists of liquid carbon dioxide almost free of methane. The fraction collecting at the top of the high pressure column contains more than 90 mole per cent of methane. It is expanded isenthalpically and then fed to a lower pressure column operating at a pressure near to but below 46.4 bar. The liquid fraction collecting at the bottom of the low pressure column contains about 12% by volume of carbon dioxide. It is pumped to the high pressure column and thus loss of product methane in this fraction is avoided. The vapour fraction collecting at the top of the lower pressure column contains only traces of carbon dioxide and is thus taken as product. It will therefore be appreciated that unlike other fractional distillation processes for separating mixtures of carbon dioxide and methane the process described by Schianni enables a relatively high purity methane product to be produced in high yield. Previous processes had either sacrificed yield for purity or purity for yield. The process proposed by Schianni is only suitable for the treatment of mixtures of carbon dioxide and methane having a relatively large carbon dioxide concentration. Schianni states that below about 30 mole per cent of carbon dioxide, the utilitites consumptions of conventional removal systems (of the adsorption kind) are less than that of his distillation process.
It can thus be seen that all the distillation processes proposed to date are for very low carbon dioxide concentrations, e.g. less than 1%, or for higher carbon dioxide concentrations, e.g. greater than 20%, or necessitate the use of an additive.
There is thus a need for an improved process and apparatus for the separation of a methane-carbon dioxide mixture containing up to about 12% by volume of carbon dioxide, and this invention aims at meeting this need.